Method for remote detection of trace contaminants

ABSTRACT

A method for remote detection of trace contaminants in a target area comprises applying sensor particles that preconcentrate the trace contaminant to the target area and detecting the contaminant-sensitive fluorescence from the sensor particles. The sensor particles can have contaminant-sensitive and contaminant-insensitive fluorescent compounds to enable the determination of the amount of trace contaminant present in the target are by relative comparison of the emission of the fluorescent compounds by a local or remote fluorescence detector. The method can be used to remotely detect buried minefields.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with Government support under contract no.DE-AC04-94AL85000 awarded by the U.S. Department of Energy to SandiaCorporation. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates to the remote detection of tracecontaminants in a target area and, more particularly, to the detectionof explosives by remote observation of the effect of explosive-signaturecompounds on the fluorescence of sensor particles applied to a suspectmined area.

Currently, there are no reliable methods for the remote detection ofburied landmines and/or antipersonnel minefields. Geophysical sensorsfor landmine detection, such as electromagnetic induction sensors,magnetometers, and radar can exhibit high probabilities of detection buttheir performance is often limited by high false alarm rates due to“clutter” from objects with physical properties similar to those of thetarget mines.

The presence of explosive compounds in or above the soil can provide auseful discriminating signature for detection of buried mines or otherunexploded ordnance. However, neither trace chemical detection methods(“sniffers”) nor bulk methods (e.g., nuclear quadrupole resonance) arecapable of standoff detection at ranges greater than a few meters.Additionally, trace chemical methods are limited by environmental fateand transport of explosive compounds in the minefield environment. Theconcentration of landmine signature compounds like trinitrotoluene (TNT)and dinitrotoluene (DNT) adsorbed on soils has been found to far exceedthe equilibrium vapor concentration, while most of the transport ofthese explosives through soil occurs in the aqueous phase. M. Ia Groneet al., SPIE 3710, 401 (1999). The maximum measured concentrations ofnitroaromatic compounds in surface soils above buried landmines havebeen reported to be as high as a part per million (about 1 mg/kg).However, laboratory measurements of vapor concentrations above similarlycontaminated soils are many orders of magnitude lower. Thus, phasepartitioning favors detection of adsorbed or aqueous compounds.Furthermore, the difficulty in detecting vapor fromadsorbed explosivesin the soil, in conjunction with the requirement for rapid sampling andanalysis to maintain forward progress of a sensor, places severerequirements on the performance of vapor sensors. In particular,extremely low mass limits of detection are required for a trace chemicalsensor to respond to the vapor over a buried landmine.

Significant investment has been made in developing remote opticalmethods (optical absorption and/or fluorescence light detection andranging) for the detection of chemical- and bio-warfare agents. However,no similar methods currently exist for the remote detection ofminefields, due to the low concentrations and lack of suitable spectralsignatures for nitroaromatic explosives. Therefore, there remains a needfor a reliable method of remote detection of buried minefields and/orantipersonnel minefields.

SUMMARY OF THE INVENTION

The present invention is directed to a method for the remote detectionof trace contaminants in a target area, comprising applying to thetarget area sensor particles capable of preconcentrating the tracecontaminant and having comprising a contaminant-sensitive fluorescentcompound with optical emission that is sensitive to the presence of thetrace contaminant; and detecting the optical emission of thecontaminant-sensitive fluorescent compound with a detection means todetermine the amount of trace contaminant present in the target area.The method further comprises adding a contaminant-insensitivefluorescent compound with optical emission that is insensitive to thepresence of the trace contaminant to the sensor particles prior toapplying the sensor particles to the target area. Thecontaminant-insensitive fluorescent compound provides an internalreference whereby the optical emission of the contaminant-sensitivefluorescent compound can be compared to the optical emission of thecontaminant-insensitive fluorescent compound to determine the amount oftrace contaminant present in the target area. The method furthercomprises exposing the target area to a mobile phase to facilitate theuptake of the trace contaminant by the sensor particles. Thefluorescence detection can be done locally or remotely. Furthermore, thedetection means can be a spectroscopic or imaging detector.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form part ofthe specification, illustrate the present invention and, together withthe description, describe the invention. In the drawings, like elementsare referred to by like numbers.

FIG. 1 shows a graph of the aqueous partition coefficients for TNT ontocandidate polymeric sensor particles and soil.

FIG. 2 shows the chemical formulas for fluorescent compounds that can beadded to the sensor particles. FIG. 2A shows the chemical formula for aniptycene-derived phenyleneethynylene polymer. FIG. 2B shows the chemicalformula for p-bis(o-methylstyryl)-benzene.

FIG. 3 shows a graph of the normalized emission spectra of sensorparticles from wetted contaminated- and uncontaminated-soil taken with alaboratory emission spectrometer.

FIG. 4 shows an elapsed time graph of the MSB/C14 emission ratio fromsensor particles applied to various soils after wetting.

FIG. 5 shows a schematic illustration of a Light Detection And Ranging(LIDAR) instrument.

FIG. 6 shows a graph of the normalized emission spectra of sensorparticles from an uncontaminated soil sample and a soil samplecontaminated with 100 ppm TNT collected with the LIDAR instrument at 500m range.

FIG. 7 shows a graph of the normalized emission spectra of sensorparticles from an uncontaminated soil sample and a soil samplecontaminated with 1 ppm TNT collected with the LIDAR instrument at 500 mrange.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relies on chemical to optical signal transductionthrough the use of sensor particles having a fluorescent signature thatchanges due to chemical reactions with the trace contaminant ofinterest. The sensor particles can be applied over the target area byair. Each sensor particle can essentially perform a chemical assay ofits local environment and selectively uptake the trace contaminant. Theuptake of the trace contaminant can thereby change the fluorescentsignature of the sensor particles, the change being capable of remoteoptical detection. Ratiometric fluorescence sensing, whereby theemission of a contaminant-sensitive fluorescent compound in the sensorparticle is compared to the emission of a contaminant-insensitivefluorescent compound in the sensor particle, enables the practicalapplication of this method for field use. For sensor particles havingfavorable spectral signatures, remote trace contaminant sensing can beachieved by laser-induced-fluorescent Light Detection And Ranging(LIDAR). The method can be applied generally to the detection of tracecontaminants in a target area. The method has been applied to thefluorescence detection of explosives in soil.

Preparation of the Sensor Particles

Sensor particles preferably both absorb the trace contaminant from thelocal environment and exhibit changes in fluorescence due to thepresence of the absorbed trace contaminant. The primary means forexplosive transport in a soil environment is in the aqueous (soil water)phase. FIG. 1 shows the aqueous partition coefficients of TNT forcandidate polymer sensor particles of polystyrene andpolystyrene-divinyl benzene (PS/DVB) copolymer, and a sieved fraction(<100 mesh) of natural soil from a target minefield. Aqueous partitioncoefficients (K_(d)) for TNT onto the candidate polymer sensor particleswere measured by agitating measured masses of particles with TNTsolutions of known initial concentration and monitoring the drop in theaqueous concentration as the mixed phases approached equilibrium. ThePS/DVB copolymer particles have a significantly larger K_(d) than soil.The PS/DVB copolymer can therefore selectively absorb and preconcentrateTNT when sufficient water is present in the soil to provide a mobilephase.

Therefore, PS/DVB copolymer particles were selected as the sensorparticles, due to their ability to absorb nitroaromatics, including TNT,from aqueous solution.

Nitroaromatic explosives exhibit strong ultraviolet absorption but lowfluorescent quantum yields, with nonradiative deexcitation of excitedstates predominating over radiative emission. Therefore, directdetection of the trace contaminants by fluorescence can be inefficient.However, the sensor particles can be doped with an appropriatefluorescent compound to sense the presence of the trace contaminant.Emission from the fluorescent compound may be enhanced or quenched dueto the presence of the trace contaminant. Synthetic fluorescent polymershave been developed whose emission is strongly quenched in the presenceof nitroaromatics. V. E. Williams et al., SPIE Proceedings 3710, 402(1999). In particular, fluorescent optical sensitivity of the sensorparticles to nitroaromatics can be achieved by dissolving a smallproportion (about 10⁻⁴ by mass) of an iptycene-derivedphenyleneethynylene polymer, hereinafter referred to as C14, into thePS/DVB copolymer particles. The C14 polymer is shown in FIG. 2A. The C14polymer exhibits a very sensitive response to the presence ofnitroaromatic explosives, due to strong oxidative quenching of thehighly delocalized excited state of the C14 polymer. When dissolved inthe PS/DVB sensor particle matrix, the C14 polymer exhibits anabsorption maximum at approximately 430 nm wavelength, and an emissionmaximum at approximately 460 nm.

Quenching of the emission of the fluorescent compound in the sensorparticle in the presence of the trace contaminant can be measuredlocally, with a field-portable spectrometer; in a laboratory; orremotely, with LIDAR. For minefield detection of explosives, remotedetection is preferable. However, standoff measurements of fluorescencefrom particles applied to the ground over a minefield present severaldifficulties. The 460 nm wavelength emission of the C14 polymer is inthe visible region of the electromagnetic spectrum, such that a changein emission intensity due to TNT-induced quenching of the sensorparticle emission would present a dark signal on a bright background.

In addition, the decrease in emission due to nitroaromatic quenchingshould preferably be resolvable from variations from other factors, suchas uneven distribution of the sensor particles. A preferred method forremote detection is to “spray” the suspected mined areas with asuspension of sensor particles in water. The water provides a mobilephase for preconcentration of the TNT in the sensor particles prior tothe fluorescence measurement. With spraying, the distribution of sensorparticles per unit area in such a mined area may not be sufficientlyuniform for landmine detection based on spatial variation of thefluorescent signal from the TNT-sensitive fluorescent compound in thesensor particle alone. Therefore, a TNT-insensitive fluorescentcompound, p-bis(o-methylstyryl)-benzene (MSB), shown in FIG. 2B, can beadded in the formulation of the sensor particles. The MSB issubstantially unaffected by the presence of the nitroaromatic tracecontaminants. In the ratiometric fluorescence sensing method of thepresent invention, the TNT-insensitive MSB emission can be used tonormalize the TNT-sensitive C14 emission signal at 460 nm for variationsin background illumination, pump illumination, and particle coverage. Inaddition to providing an internal reference standard, MSB provides forefficient energy transfer of the excitation radiation to theTNT-sensitive fluorescent compound, C14. This is because MSB has anabsorption maximum in PS/DVB at about 360 nm wavelength and exhibitsmultiple emission maxima at about 400 nm and 420 nm wavelengths, nearthe excitation maximum for C14.

Sensor particles for TNT detection can be prepared from 200-400 mesh,80% styrene/20% divinylbenzene copolymer beads. 600 μl of chloroform,100μl of 0.338 mg/ml C14 polymer in toluene, and 36 μl of 25 μg/ml MSBin toluene can be combined with 0.3 g of PS/DVB beads in a glassscintillation vial, which can then be capped and allow to standovernight. After this mixing and incubation time, the processingsolvents can be removed in a rotary evaporator. Once the mixture hasdried to the point where free solid clumps of material are observed, thevial can be removed from the rotary evaporator, two 1/8″ steel balls canbe added, and a small amount (0.2-0.5 ml) of chloroform can be used towash material from the sides of the vial. The rotary evaporation stepcan be repeated. When the material in the vial appears to be dry, thevial can be removed from the rotary evaporator, capped and shaken withthe steel balls to break up agglomerated particles. The processedmaterial can be visually inspected under ambient light and ultravioletlight for uniformity of color and particle size. Following visualinspection, fluorescence emission spectra can be obtained to verify theproper MSB/C14 ratio.

TNT and DNT-contaminated natural soil samples were prepared to evaluatethe detection method of the present invention using sensor particlesprepared by the above described process. Samples of uncontaminatedsurface soil were collected and sieved through a 2-mm screen and driedat 140° C. for 7 days. Analytical grade DNT crystals were added to thesoil in a one-gallon paint can to a concentration of about 10 mg/kg. Thecontainer was placed on a roller mill for 4 is days at about 30 rpm.Water was added to bring the soil to about 0.01 g water/g soil, and theDNT-contaminated soil samples were placed in an oven at 110° C. for 4days to distribute the explosive. The container was then removed fromthe oven and placed on the roller mill for 1 day. Recrystallizedmilitary grade TNT was used to prepare stock TNT-contaminated soilsamples in a similar fashion, to a concentration of about 100 mg/kg. Tocreate lower soil concentrations, the stock soil was serially dilutedwith clean dry soil. The contaminated soils were assayed afterpreparation using solvent elution and gas chromatography, to verifyconcentration and homogeneity.

The sample holders for the evaluations were simple aluminum cylinders,2.5 inches in diameter and 1 inch deep, with a 2-inch diameter uv quartzwindow on one end face and a removable cover on the other. For practicaluse of this method in the field, the mass of sensor particles appliedper unit area across a suspected minefield should preferably be keptlow. A concentration of 1 mg of sensor particles per square centimeterresults in a reasonable total mass requirement of 100 kg of sensorparticles per hectare of target area (100 kg/10,000 m²). Therefore,contaminated samples were prepared by sprinkling about 1 mg/cm² of thesensor particles by hand on the inner face of the quartz window.Approximately 10 g of soil was then placed in the sample holder.

Local Fluorescence Detection

FIG. 3 shows normalized fluorescence spectra from contaminated anduncontaminated soils obtained with a conventional single-gratingemission spectrometer having an arc lamp excitation source. Theexcitation light at 350 nm was normally incident (90 degrees from thesample plane) and fluorescence was collected at an angle of 22.5 degreesfrom the surface normal. The spectra were taken after both contaminatedand uncontaminated soil samples were wetted with about 0.3 g HPLC gradewater per gram of soil. The spectra indicate quenching of the emissionof the C14 polymer at 460 nm and 490 nm relative to the unquenched MSBemission peaks at 400 nm and 420 nm, due to the presence of TNT in thecontaminated sample. Similar spectra acquired for sensor particles oncontaminated but dry soil 15 samples did not exhibit significant C14quenching. Thus, the presence of water as a mobile phase can facilitatetransport of the explosive from the surface of the soil to the sensorparticles. FIG. 4 shows the ratio of intensities of the MSB emission at420 nm to the emission of the C14 polymer at 460 nm, plotted as afunction of elapsed time following addition of water to the soilsamples. This MSB/C14 emission ratio increases with elapsed time afterwetting, as the TNT is transported from the soil through the water phaseto the sensor particles, thereby causing the TNT to quench the C14emission. The maximum increase in the signal occurs within 2-3 minutesafter water is added to the system and reaches an asymptotic valuewithin approximately 1 hour, indicating the rapid transport of TNT inthe aqueous phase.

Remote Fluorescence Detection

FIG. 5 shows a schematic illustration of the LIDAR instrument 50 thatcan be used to obtain remote fluorescence measurements of thecontaminated soils 58 by UV laser-induced fluorescence. The LIDARinstrument 50 comprises a tunable UV laser source 51, a collectiontelescope 52, and a range- and spectrally-resolved detector 53. R. M.Measures, Laser Remote Sensing: Fundamentals and Applications,Wiley-Interscience (New York) 1984. The fluorescent light 55 from thesensor particles containing the mixed MSB+C14 compounds can be opticallypumped with a 355 nm wavelength excitation pulse 54 from a tripledNd:YAG laser 51. The excitation pulse 54 can be and transmitted to thecontaminated soil 58 coaxially with the receiver field of view. TheNd:YAG laser 51 provides for a relatively lightweight, compact, rugged,pulsed excitation source for interrogation of the sensor particlefluorescence in the field. Backscattered light and fluorescent returnlight 55 can be gathered by the collection telescope 52 comprising an80-cm-diameter primary mirror that collimates the collected return light55 to an about 2-cm-diameter beam. Two uncoated quartz beamsplitters 56send a fraction (about 15%) of the return light 55 to twophotomultiplier tubes 57 which can provide time-resolved elastic andfluorescence data.

The remainder of the collected return light 55 can be sent to thespectrometer 53 with an charge-coupled-device (CCD) detector to allowtime-gated spectrally resolved analysis of the sensor particlefluorescence. For the detection of the explosive-contaminated soils, theCCD detector can be electronically gated with a gate width of 100 ns anda starting gate coincident with the prompt scattered light return, dueto the short emission lifetime of the MSB and C14 fluorescence. Along-pass optical filter can be used to block the elastically scatteredlight from the spectrometer and enhance the signal-to-noise of thelaser-induced-fluorescence light. Typically, from 300 to 1000 excitationlaser shots can be integrated on the CCD detector before readout. Theexcitation laser pulse rate can be 30 Hz, resulting in integration timesof 10-33 seconds per collected spectrum.

To evaluate the method of the present invention for remote detection inthe field, contaminated soil samples and uncontaminated samples, withsensor particle coverage of about 1 mg/cm², and which were wetted withabout 0.3 g water/gram soil, were placed at a standoff range of 500meters from the LIDAR instrument 50. The laser excitation beam 54 of theLIDAR instrument 50 has a nominal diameter of approximately 8 inches at500 m range. At this range, the excitation beam 54 had a mean of 2.1 mJper excitation pulse incident on the 2″ diameter soil samples.

FIG. 6 compares normalized spectra obtained from an uncontaminated soilsample and a contaminated soil sample, containing 100 ppm TNT.Backscattered incident light and the background spectra from pure soilwere subtracted from both samples. The key fluorescence emission peaksat 420 nm (due to the MSB) and 460 nm (due to the C14 polymer) are botheasily resolved. The decrease in the C14 emission peak at 460 nmrelative to the MSB emission peak at 420 nm indicates the quenching ofthe C14 emission due to TNT. Thus, the presence of 100 ppm TNT in thecontaminated soil samples can be easily detected by the remote LIDARinstrument.

FIG. 7 shows similarly normalized emission spectra for an uncontaminatedsoil sample and a contaminated soil sample, containing 1 ppm TNT.Statistical analysis of the ratios of the integrated intensity in theMSB and C14 emission peaks indicate that contaminated soils containing 1ppm TNT can be remotely detected by the LIDAR instrument at the 98%confidence level.

The LIDAR instrument 50 is spectrally resolving, but is not an imaginginstrument. Signal-to-noise (SNR) calculations were performed for animaging fluorescence instrument, using parameters for a commerciallyavailable intensified CCD array detector and laser excitation source,with a 30 cm fluorescence collection aperture. An interference filterwith a 10 nm bandpass centered at 460 nm can be used to enhance the SNRfor emission from the C14 polymer in the sensor particles. In addition,the time-gated fluorescence return light can be integrated from about1000 excitation pulses of a 30 mJ/pulse laser at 355 nm. With anexcitation pulse rate of 30 Hz, the integration time can be about 30seconds. The imaging fluorescence instrument can have a useful SNR at astandoff range of a kilometer, for images of a 50 m diameter target areawith a 5 cm nominal pixel resolution at the target. This pixelresolution is expected to be necessary to resolve signatures ofantipersonnel mines. A stationary or a ground-vehicle mounted opticalmeasurement platform designed to acquire fluorescence images at moremodest standoff range (e.g., 10 m) can have much less severe optical andtracking design requirements than the long-range imaging fluorescenceinstrument described above.

The embodiments of the present invention have been described as a methodfor remote detection of trace contaminants in a target area. It will beunderstood that the above description is merely illustrative of theapplications of the principles of the present invention, the scope ofwhich is to be determined by the claims viewed in light of thespecification. Other variants and modifications of the invention will beapparent to those of skill in the art.

We claim:
 1. A method for the remote detection of trace contaminants ina target area, comprising: a) applying sensor particles to the targetarea, the sensor particles capable of preconcentrating the tracecontaminant and having a contaminant-sensitive fluorescent compound withoptical emission that is sensitive to the presence of the tracecontaminant and a contaminant-insensitive fluorescent compound withoptical emission that is insensitive to the presence of the tracecontaminant; b) detecting the optical emission of thecontaminant-sensitive fluorescent compound and the optical emission ofthe contaminant-insensitive fluorescent compound with a detection means;and c) comparing the optical emission of the contaminant-sensitivefluorescent compound to the optical emission of thecontaminant-insensitive fluorescent compound to determine the amount oftrace contaminant present in the target area.
 2. The method of claim 1,further comprising exposing the target area to a mobile phase prior tostep b).
 3. The method of claim 2, wherein the mobile phase is water. 4.The method of claim 1, wherein the contaminant-sensitive fluorescentcompound comprises an iptycene-derived phenyleneethynylene polymer. 5.The method of claim 2, wherein the contaminant-insensitive fluorescentcompound comprises p-bis(o-methylstyryl)-benzene.
 6. The method of claim1, wherein the target area comprises soil.
 7. The method of claim 6,wherein the sensor particles comprise styrene/divinylbenzene copolymer.8. The method of claim 1, wherein the sensor particles comprise polymer.9. The method of claim 1, wherein the trace contaminant comprises anitroaromatic compound.
 10. The method of claim 9, wherein the tracecontaminant comprises trinitrotoluene or dinitrotoluene.
 11. The methodof claim 1, wherein the detection means is a laser-induced-fluorescenceinstrument.
 12. The method of claim 11, wherein the detection means is alight detection and ranging instrument.
 13. The method of claim 11,wherein the detection means is an imaging fluorescence instrument.